A household electric appliance having an electronic control system normally needs a stable DC power supply. The DC power is often directly converted from an AC power source through a power conversion device, such as a low frequency linear transformer, a switching mode power supply or a simple step down circuit. The electronic control system may need different amount of power under different working conditions and the power supply should be able to provide sufficient power to the control system all the time for the electronic control system to work properly. In a conventional technology, the power supply constantly outputs the maximum power that the control system possibly needs regardless of the actual working condition that the control system is under, which leads the power supply circuit to consume a great amount of power most of the time. Currently, some switch mode power supplies have made some improvement in this aspect, but the costs for such systems are still very high.